It is known in the art to create a digital recording medium comprising a substrate having a layer of a colored, photobleachable composition thereon. Information is recorded on the recording medium by selectively exposing portions of the recording medium to light to thereby initiate a chemical reaction which results in decomposition of the coloring agent contained therein. Examples of such prior art recording media are disclosed in U.S. Pat. No. 5,312,713; U.S. Pat. No. 4,954,380 and Japanese patent application No. 01-342989 (the disclosures of which are all incorporated herein by reference). Examples of such digital recording media include, optical disks, such as compact discs, which are a read-only, non-erasable media for storing information, such as digitized music, video, computer data, and combinations thereof, and writable optical disks, such as write once, read many times "WORM disks."
U.S. Pat. No. 5,312,713 relates to an information recording medium, such as an optical disk. The patent discloses a substrate upon which is disposed a recording layer. The recording layer comprises a mixture of organic polysilane and an oxo metallic phthalocyanine dye. An ultraviolet light source is selectively irradiated on portions of the recording layer. The irradiation causes a photo decomposition of the organic polysilane. Then, the entire recording layer is heated to a temperature equal to or greater than the glass transition point of the organic polysilane so that the decomposition product produced by the photodecomposition contacts the oxo metallic phthalocyanine pigment which causes the decoloring reaction of the pigment. Thereby, only the portion of the pigment in the recording layer which was irradiated by the ultraviolet light is decolorized; the non-irradiated portion retains it color. The information recorded on the recording layer can be read by detecting the difference among the absorbency of each portion (i.e., between irradiated and non-irradiated portions) by scanning the recording layer with low-energy laser beams.
U.S. Pat. No. 4,954,380 relates to an optical recording medium. This patent discloses a transparent substrate upon which is coated an optical recording layer which includes a bleachable organic dye which is bleachable under ultraviolet radiation, such as cyanine dyes, xanthene dyes and azine dyes. A photomask having a transparent tracking pattern is then placed over the recording layer and the mask is irradiated with ultraviolet light. The exposed portion of the recording layer is bleached due to photochemical decomposition of the organic dyes in the recording layer. As a result, there is formed in the recording layer a tracking region having different optical characteristics from the non-exposed region.
Japanese patent application No. 01-342989 relates to an optical recording medium. The recording medium comprises a base, a recording layer, a reflective layer and a protective coating layer. The recording layer comprises a coloring matter, such as a cyanine dye having a maximum absorbency of 600-900 nm, and a photobleachable coloring matter, such as an azo dye having a maximum absorbency of 350-600 nm.
A major problem with colorants used in information storage media, such as optical disks, is that they tend to fade when exposed to sunlight or artificial light. It is believed that most of the fading of colorants when exposed to light is due to photodegradation mechanisms. These degradation mechanisms include oxidation or reduction of the colorants depending upon the environmental conditions in which the colorant is placed. Fading of a colorant also depends upon the substrate upon which they reside.
Product analysis of stable photoproducts and intermediates has revealed several important modes of photodecomposition. These include electron ejection from the colorant, reaction with ground-state or excited singlet state oxygen, cleavage of the central carbon-phenyl ring bonds to form amino substituted benzophenones, such as triphenylmethane dyes, reduction to form the colorless leuco dyes and electron or hydrogen atom abstraction to form radical intermediates.
Various factors such as temperature, humidity, gaseous reactants, including O.sub.2, O.sub.3, SO.sub.2, and NO.sub.2, and water soluble, nonvolatile photodegradation products have been shown to influence fading of colorants. The factors that effect colorant fading appear to exhibit a certain amount of interdependence. It is due to this complex behavior that observations for the fading of a particular colorant on a particular substrate cannot be applied to colorants and substrates in general.
Under conditions of constant temperature it has been observed that an increase in the relative humidity of the atmosphere increases the fading of a colorant for a variety of colorant-substrate systems (e.g., McLaren, K., J. Soc. Dyers Colour, 1956, 72, 527). For example, as the relative humidity of the atmosphere increases, a fiber may swell because the moisture content of the fiber increases. This aids diffusion of gaseous reactants through the substrate structure.
The ability of a light source to cause photochemical change in a colorant is also dependent upon the spectral distribution of the light source, in particular the proportion of radiation of wavelengths most effective in causing a change in the colorant and the quantum yield of colorant degradation as a function of wavelength. On the basis of photochemical principles, it would be expected that light of higher energy (short wavelengths) would be more effective at causing fading than light of lower energy (long wavelengths). Studies have revealed that this is not always the case. Over 100 colorants of different classes were studied and found that generally the most unstable were faded more efficiently by visible light while those of higher lightfastness were degraded mainly by ultraviolet light (McLaren, K., J. Soc. Dyers Colour, 1956, 72, 86).
The influence of a substrate on colorant stability can be extremely important. Colorant fading may be retarded or promoted by some chemical group within the substrate. Such a group can be a ground-state species or an excited-state species. The porosity of the substrate is also an important factor in colorant stability. A high porosity can promote fading of a colorant by facilitating penetration of moisture and gaseous reactants into the substrate. A substrate may also act as a protective agent by screening the colorant from light of wavelengths capable of causing degradation.
The purity of the substrate is also an important consideration whenever the photochemistry of dyed technical polymers is considered. For example, technical-grade cotton, viscose rayon, polyethylene, polypropylene, and polyisoprene are known to contain carbonyl group impurities. These impurities absorb light of wavelengths greater than 300 nm, which are present in sunlight, and so, excitation of these impurities may lead to reactive species capable of causing colorant fading (van Beek, H.C.A., Col. Res. AppI., 1983, 8(3), 176).
Therefore, for all of these reasons, there exists a great need for a digital information recording medium and for a method of recording digital information on a recording medium which medium is more stable to the effects of both sunlight and artificial light.